Add Hover, Focus, and Pressed states to MDC Card

[lynnmercier]

MDC Cards should be clickable, and should use our states subsystem to show hover, focus, and pressed states

[kfranqueiro]

Is this something that should happen for all cards, or should it be either opt-in or opt-out? (e.g. I'm planning to add an opt-out class for mdc-list-item when I cover lists.)

[lynnmercier]

As discussed in sprint planning, it will be optional. An opt-in.